1. Field of the Invention
This invention relates to thin film structures for magnetic recording heads and more particularly, to laminated thin film structures and methods for forming such structures.
2. Description of the Prior Art
In conventional thin film inductive recording heads, the soft magnetic films (e.g., NiFe) are deposited using a process which causes the magnetic easy axis to be parallel to the plane of the recording medium. This usually results in triangular magnetic domains formed along left and right edges of the soft film which are called closure domains. The closure domains form in order to minimize the total magnetic energy associated with the film and its surrounding space. Such closure domains lower the total energy despite the fact that the magnetization is directed completely or partially in the hard magnetic axis direction.
When the head is driven by a current loop, or when the head is sensing a magnetic field from the recording medium, the net magnetic induction in the soft film of the head increases in the vertical direction by rotation of the magnetization vector in the central domains, along with alternate growth and shrinkage of closure domains. By convention, a recording head is regarded as lying in a vertical plane above a horizontal recording medium so that the vertical direction is axial to the pole tips and normal to the medium. The presence of the side closure domains causes wall motion to accompany magnetization rotation, thereby limiting permeability and head efficiency. In addition, noise can be created whenever defects interact with the walls or conversion of the domain structure to a new pattern occurs.
When the throat or poletip region of an inductive head, or the flux guide for a magnetoresistive head, is made narrow horizontally, to achieve a narrow recording trackwidth, the closure domains become more significant. In this case, when the head is driven by a current loop or senses an external field, magnetization in the vertical direction increases mainly by horizontal motion of the vertical wall in the center of the poletip. While the mobility of this wall may be sufficient to give adequate head efficiency, large displacements of such a wall may cause the wall to encounter a wall pinning defect, leaving the head domain configuration in a mestastable state. This can result in changes of reading efficiency during operation and wall-motion (Barkhausen) noise for narrow track heads or for perpendicular recording "probe" heads whose throat height to throat width ratio favors vertical wall formation.
To make a high efficiency, low noise, narrow track head, it is desirable to force the magnetization into the horizontal easy axis direction, and to eliminate the side closure domains. One prior art technique for partially accomplishing this is to laminate the soft magnetic films used for the yoke of the head. Instead of depositing one layer, a plurality of magnetic layers are deposited with each pair of magnetic layers being separated by a thin nonmagnetic spacer layer. Each magnetic sublayer may then have its magnetization lie in the easy axis direction, but neighboring layers have their magnetization directed horizontally antiparallel. Flux closure between the layers is through the spacer near the edges and via external fringe fields at the side edges.
Slonczewski, et al., "Micromagnetics of Laminated Permalloy Films", IEEE Trans. on Mag., Vol. 24, No. 3, p. 2045, May 1988, show that in laminated films of the proper dimensions, closure domains are replaced by edge-curling walls. The interior of the film has a magnetization aligned in the easy axis direction which is optimum for transmitting flux by magnetization rotation. However, Herman, et al., "Study of Field-Driven Wall-Configuration Conversions for Laminated Permalloy in the Easy-Axis State", J. Appl. Physics, Vol. 63, No. 8, p. 4036, April 1988, and "Edge-Curling-Wall Discontinuities and Interactions with Bloch Walls in Easy-Axis Permalloy", IEEE Trans. on Mag., Vol. 24, No. 6, p. 3066, November 1988, showed that though a single domain (no wall) state can occur in a laminated film recording head yoke, the most stable state is found to have a single wall in each magnetic layer. This wall can be a source of instability and noise. In addition, the edge-curling walls reduce the active cross-section of the film and can also be a source of noise. Reducing the width of the edge-curling walls demands multiple thin laminations with very thin nonmagnetic spacers which require additional, carefully controlled fabrication steps. Moreover, in addition to extra expense, multiple thin laminations are potentially subject to problems such as spacer pinholes and higher coercivity. C. Tsang, et al., "Magnetics of Nonlaminated, Bilaminated, and Multilaminated Permalloy Stripes", J. Appl. Physics, Vol. 63, No. 8, p. 2938, April 1988 have shown that magneto-resistance measurements of narrow, multiple laminated films show noise and instabilities due to additional complex domain states. Thus, while a simple lamination is adequate to eliminate most noisy domain walls from a wide, about 100 um, yoke of the head, it is very difficult to fabricate narrow, less than 10 um, poletip regions of flux guides with the necessary multiple thin laminations.
In another approach, disclosed in U.S. Pat. No. 4,103,315, the domain walls are minimized by a multiple thin film structure including at least one pair of layers of a ferromagnetic material and an antiferromagnetic material deposited one upon one another, that are exchange coupled to retain a unidirectional bias in the plane of the ferromagnetic material. If multiple pairs of layers are used, a layer of nonmagnetic material is provided to separate the pairs. The successive pairs of layers have their unidirectional bias pointing in opposite directions. However, if the bias is strong, rotational permeability and head efficiency are reduced. If the bias is weak, domain walls are not completely eliminated and the films exhibit Barkhausen noise.